Power converter circuit board

ABSTRACT

Prior power converters have utilized discrete components mounted on circuit boards of different design. Failure of one or more of the components requires the components to be separately tested to determine which is faulty. The faulty component or the board containing same must then be replaced. This results in significant down time for the inverter and can require stocking of a large number of specialized boards. In order to overcome these problems, an inverter is assembled using a series of circuit boards of standardized type. Each circuit board includes separate layers which interconnect components mounted thereon together with a heat exchanger which cools high power components. When a malfunction of a component occurs, the entire board may be replaced so that inverter down time is minimized. Also, the use of standardized boards reduces stocking requirements and inventory costs.

TECHNICAL FIELD

The present invention relates generally to power converters, and moreparticularly to a circuit board for a power converter.

BACKGROUND ART

In a variable-speed, constant-frequency (VSCF) power generating system,variable-frequency power produced by a brushless, synchronous generatoris converted by a power converter into constant-frequency AC power. Thepower converter includes a rectifier bridge which converts thevariable-frequency power into DC power on DC link conductors and aninverter which converts the DC power into the constant-frequency ACpower. The inverter may be, for example, of the neutral point clampedtype wherein positive and negative switch assemblies are connected inseries across the DC link conductors and wherein a neutral switchassembly is connected between a neutral voltage and a junction betweenthe positive and negative switch assemblies.

In general, each of the positive and negative switch assemblies includesdriver and driven transistors connected together in a Darlingtonconfiguration together with associated base biasing, snubber and flybackcomponents connected thereto. The neutral switch assembly includes allof these components with the exception of a flyback diode across thedriven transistors. Also, the neutral switch assembly requires four highpower diodes connected in a bridge configuration which permitbi-directional current flow between the neutral voltage and the phaseoutput. The neutral switch assembly thus requires three additional powerdiodes as compared with each of the positive and negative switchassemblies.

The components of each switch assembly handle large magnitudes ofcurrent and are subject to failure. Such a failure may cause harmoniccontent in the inverter output to increase beyond acceptable limits ormay render the entire inverter inoperative. In such a case, it isnecessary to shut down the inverter in order to isolate and replace thefailed component. The process of isolating the failed component can belengthy, in turn leading to significant down time for the inverter.

The process of isolating a failed component can be shortenedconsiderably by packaging each of the switch assemblies as a separateunit, such as on a single printed circuit board. Thus, for each inverterleg, only three components, (i.e. each switch assembly) need be testedin order to isolate a failed component. Once the failure is isolated toa single board, the entire board can be replaced and the faulty boardrepaired "off-line". While this approach minimizes the time that aninverter is out of service, it requires that at least two different setsof boards be used, one for the positive and negative switch assembliesand one for the neutral switch assembly. This, in turn, increases thenumber of boards which must be stocked for replacement purposes.

SUMMARY OF THE INVENTION

In accordance with the present invention, an inverter includes aplurality of switch assemblies formed by components mounted onstandardized circuit boards.

Generally, a circuit board according to the present invention comprisesa substrate having a main layer including an electrically conductiveportion and an electrically non-conductive portion adjacent thereto, thesubstrate having first and second opposite faces wherein theelectrically conductive and non-conductive portions extend substantiallythe entire distance from the first face to the second face. A layer ofinsulating material is disposed on one side of the first and secondfaces and an electrically conductive component, such as a conductivetrace, is disposed on the layer of insulating material. Means includingvia holes extend through the layer of insulting material for couplingthe electrically conductive component to the electrically conductiveportion of the substrate.

In a more specific sense, the circuit board main layer includes aplurality of spaced copper sections with electrically non-conductivefiller therebetween. Electrically insulating layers are deposited onfirst and second sides of the main layer. One or more planar thermofoilresistors are disposed on one of the insulating layers and a furtherelectrically insulating layer is deposited on the resistors. Anelectrically conductive heat exchanger is disposed on the last-namedinsulating layer.

Disposed on the other insulating layer is a series of traces comprisingprinted strips of electrically conductive material. A still furtherinsulating layer is deposited atop the trace layer. Via holes andapertures are formed in the various layers to allow connection tovarious components. The assembled components described above form acircuit board blank and components may be added to three of the blanksto form positive, negative and neutral switch assemblies.

The design of the board permits the positive, negative and neutralswitch assemblies to be stacked next to one another so that the size ofthe resulting inverter is minimized. Cooling fluid is provided to theheat exchangers so that heat is effectively removed even from componentslocated in the inner board or boards intermediate the ends of the stack.Thus, a large number of heat-producing components can be assembledwithin a small space without damage due to heat build-up.

A faulty inverter due to failure of a component on a particular boardcan be easily repaired by appropriately configuring a blank board and bysubstituting the configured board for the board containing the failedcomponent. Since the positive, negative and neutral switch assembliesare assembled using identical board blanks, there is no need tomanufacture and stock different board types to allow any switch assemblyto be replaced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a block diagram of a variable-speed, constant frequencypower generating system in conjunction with a prime mover;

FIG. 2 comprises a block diagram of the inverter shown in FIG. 1;

FIG. 3 comprises a block diagram of one leg of the inverter shown inFIG. 2;

FIG. 4 comprises a schematic diagram of power switches together withbase biasing, snubber and flyback circuitry of the inverter leg shown inblock diagram form in FIG. 3;

FIG. 5 is a perspective view of a first side of the circuit board of thepresent invention configured to form a positive switch assembly;

FIG. 6 is a perspective view of a second side of the circuit board ofFIG. 5;

FIG. 7 comprises an exploded perspective view of the circuit board ofFIG. 5 illustrating various electrical components separated from thecircuit board;

FIG. 8 comprises an exploded perspective view from the first side of thecircuit board of FIG. 5 illustrating several of the layers thereof;

FIG. 9 comprises an exploded perspective view from the second side ofthe circuit board of FIG. 5 illustrating the remainder of the layersthereof;

FIG. 10 comprises a plan view of the first side of the circuit board ofFIG. 5;

FIG. 11 comprises a plan view of the first side of the circuit board ofthe present invention configured to form a negative switch assembly; and

FIG. 12 comprises a plan view of the first side of the circuit board ofthe present invention to form a portion of a neutral switch assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a variable-speed, constant-frequency (VSCF)power generating system 10 converts variable-speed motive power producedby a prime mover 12, such as an aircraft jet engine, intoconstant-frequency AC electrical power on a load bus 14. It should benoted that various contactors which connect the VSCF system 10 to theload bus 14 are not shown for the sake of simplicity.

The VSCF system 10 includes a brushless, synchronous generator 16 whichconverts the variable-speed motive power produced by the prime mover 12into variable-frequency AC power. This AC power is converted by an AC/DCconverter and transient suppressor 18 into DC power on a DC link 20including DC link conductors 20a, 20b. A DC link filter (not shown) maybe provided to minimize ripple in the DC power on the DC link 20. Theinverter 22 converts the DC power into constant-frequency AC power whichis filtered by an optional filter 24 and provided to the AC bus 14.

Referring now to FIG. 2, the inverter 22 is preferably, although notnecessarily, of the three-phase type including first through thirdinverter legs 30a, 30b and 30c which are coupled to the DC linkconductors 20a, 20b and a neutral terminal 32 at which a neutral voltageis produced. The neutral terminal comprises the junction between firstand second capacitors C1 and C2 coupled across the DC link conductors20a, 20b.

FIG. 3 illustrates the inverter leg 30a in greater detail, it beingunderstood that the legs 30b and 30c are identical thereto. Positive andnegative switch assemblies 34 and 36 are coupled in series across the DClink conductors 20a, 20b. A phase output is produced at a terminal 38which comprises the junction between the switch assemblies 34 and 36. Aneutral switch assembly 40 is coupled between the phase output terminal38 and the neutral terminal 32.

The switch assemblies 34, 36, 40 are operated to produce an AC waveformat the output terminal 38 comprising a series of alternating positiveand negative half-cycles. The positive half-cycle is produced byalternately operating the switch assemblies 34 and 40 in a pulse-widthmodulated (PWM) mode. Each negative half-cycle is produced byalternately operating the switch assemblies 36 and 40 in a PWM mode. Theoutput voltage produced at the terminal 38 thus switches between apositive or negative level and the neutral voltage, but not between thepositive and negative levels. The inverter is thus referred to as beingof the neutral point clamped type. It should be noted that inverter 22need not be of this type, in which case the neutral switch assembly 40of the leg 30a and the corresponding neutral switch assemblies of thelegs 30b and 30c would not be needed. In this case, the switchassemblies 34 and 36 may be alternately operated in a PWM mode or may beoperated to produce a stepped output waveform, if desired.

Referring now to FIG. 4, the positive switch assembly 34 includes adriver transistor QD+ which is connected in a Darlington configurationwith driven transistors Q1+ and Q2+. Connected to the bases of thetransistors QD+, Q1+ and Q2+ is base biasing circuitry including diodesD1+ through D3+ and DB1+ through DB3+ together with biasing resistorsRB1+ and RB2+. An additional diode DB4+ together with a zener diode ZB1+are connected to the bases of the transistors Q1 and Q2.

Connected across the collectors and emitters of the driven transistorsQ1+ and Q2+ are flyback and snubber circuits including a flyback diodeDF2+, a snubber diode DS+, snubber resistors RS1+, RS2+ and snubbercapacitors CS1+, CS2+.

The negative switch assembly 36 includes identical components describedabove in connection with the positive switch assembly 34. In order todistinguish between the components of the negative switch assembly andthe components of the positive switch assembly, the plus signs in thereference designations for the components of the positive switchassembly 34 are replaced by negative signs for the components of thenegative switch assembly 36.

The neutral switch assembly 40 includes base biasing components D1Nthrough D3N, DB1N through DB4N, RB1N, RB2N and ZB2N, power switchingcomponents including a driver transistor QDN and driven transistors Q1N,Q2N and snubber components DSN, RS1N, RS2N, CS1N and CS2N. Thesecomponents correspond to the components D1+ through D3+, DB1+ throughDB4+, RB1+, RB2+, ZB2+, QD+, Q1+, Q2+, DS+, RS1+, RS2+, CS1+, and CS2+,respectively, of the positive switch assembly 34. In addition, theneutral switch assembly 40 includes diodes DN1+, DN1-, DN1N and DN2N,snubber resistors RDN2+, RDN1-, RDN1N and RDN2N and snubber capacitorsCDN2+, CDN1-, CDN1N and CDN2N.

Current flow between the neutral terminal 32 and the phase output 38 isaccomplished in a bi-directional fashion utilizing the diodes DN1+,DN1-, DN1N and DN2N, together with the switches QDN, Q1N and Q2N. Morespecifically, current flow from the phase output 38 to the neutralterminal 32 occurs through the diode DN1+, the transistors QDN, Q1N andQ2N and the diode DN2N. Conversely, current flow from the neutralterminal 32 to the phase output 38 takes place through the diode DN1N,the transistors QDN, Q1N and Q2N and the diode DN1-.

The switches of each switch assembly 34, 36 and 40 are turned on bypositive base currents supplied over lines 64, 66 and 68, respectively.The switches are turned off by drawing current out of the bases of thesetransistors over the lines 64, 66 and 68 and through additional lines70, 72 and 74 which are coupled to the bases of the driven transistorsQ1+ and Q2+, Q1- and Q2- and Q1N and Q2N, respectively. Groundreferences for the switch assemblies 34 and 40 are provided over lines77 and 79 while a ground reference is provided for the switch assembly36 by the DC link conductor 20b. It should be understood that thecircuitry for turning the transistors of these switch assemblies on andoff is unimportant to an understanding of the present invention, andhence will not be described in detail herein.

The components of FIG. 4 may be packaged as three separate units asillustrated by the dashed lines of FIG. 4 which in turn leads to adesirable standardization of the units. More particularly, the diodeDN1+, resistor RDN2+ and capacitor CDN2+ may be packaged on a circuitboard 80 together with the power switches and the base biasing, snubberand flyback circuits of the switch assembly 34. The diode DN1-,capacitor CDN1- and resistor RDN1- may be packaged on a circuit board 82together with the power switches and the base biasing, snubber andflyback circuits of the negative switch assembly 36. The diodes DN1N,DN2N, the capacitors CDN1N and CDN2N and the resistors RDN1N and RDN2Nmay be packaged on a single circuit board 84 with the power switches andthe base biasing and snubber circuits of the neutral switch assembly 40.Inasmuch as the neutral switch assembly 40 does not require a flybackdiode, it can be seen that such a packaging arrangement results in anidentical number of components on each circuit board 80, 82 and 84,except that the board 84 includes an extra resistor and capacitor ascompared with the boards 80 and 82. This arrangement of componentsallows the design of the board to be standardized so that a single boardblank can be configured to form a positive switch assembly, a negativeswitch assembly or a portion of a neutral switch assembly, as desired.

FIG. 5 illustrates the circuit board 80 in detail from a first or a topside thereof while FIG. 6 illustrates the board from a second or lowerside thereof. As seen in these FIGS., the board includes a substrate 100which is bonded to a heat exchanger 102. The heat exchanger 102 iselectrically and thermally conductive and is preferably fabricated ofaluminum. The heat exchanger 102 forms a collector bus for the switchassembly 34. As seen specifically in FIG. 7, the diodes DS+, DF2+ andDN1+ are disposed in apertures 103a-103c, respectively, through thesubstrate 100 in thermal contact with a first side of the heat exchanger102 while the transistors QD+, Q1+ and Q2+ are disposed on in electricaland thermal contact with a second side of the heat exchanger 102. Thediodes DS+ and DF2+ are directly electrically connected to the collectorbus formed by the heat exchanger 102. As seen in the exploded view fromthe top side of FIG. 7, a conductive pad 104 and an electricallyinsulating Kapton spacer 105 are disposed between the anode of the diodeDN1+ and the heat exchanger 102 so that the anode can be connected toother components in a fashion hereinafter described. "Kapton" is aregistered trademark of E.I. duPont de Nemours and Company for flexiblefilm for electrical insulation.

The diodes D1+ through D3+, DB1+ through DB4+ and ZB1+, the resistorsRB1+ and RB2+ and the capacitors CS1+, CS2+ and CDN2+ are mounted on thesubstrate 100. The resistors RS1+, RS2+ and RDN2+ comprise planar etchedfoil resistors which are formed as a layer by an etching process as partof the substrate 100 and are shown in FIG. 9. A series of conductivetraces forming a trace layer 106 and a series of via holes 107 (severalof which are identified in FIG. 7) are provided as a part of thesubstrate 100. The substrate 100 further includes a main layer 101having three buses comprising an emitter bus 108, an input/output bus109 and first and second capacitor buses 110, 112. Spaces between thebuses 108, 109, 110 and 112 are occupied by electrically insulativefiller material 114 and the portions are bonded together by a bondingagent, such as epoxy, to form the main layer 101. The filler material114 may be a commercially available product known commonly as G10material which conforms to military specification MILP13949.

The trace layer 106 includes a series of traces 120, 122, 124, 126, 128,130, 132, 134 and 136 which, as noted in greater detail hereinafter, aredeposited on the main layer 101, again by an etching process. The viaholes 107, the traces 120-136, the buses 108, 109, 110 and 112, the heatexchanger 102, the conductive pad 104, additional conductive pads 140,142 and electrically conductive plates 144, 146 and 148 interconnect thevarious components on the board 80. Various junction points in theschematic of FIG. 4 are identified by a number preceded by the letter Eand are shown in FIGS. 9 and 10. The schematic of FIG. 4 also includesreference numerals identifying the interconnection of the components onthe board by the conductive pads 104, 140, 142, the heat exchanger 102,the conductive traces 120-136, the buses 108, 109, 110 and 112 and theplates 144, 146 and 148 as well as conductive mounting posts hereinafterdescribed.

More particularly, the trace 122 couples connection points E1 and E2 tothe anodes of the diodes D1+ and D2+ and to the cathode of the diodeD3+. The trace 124 couples the cathode of the diode D1+ to a connectionpoint E19 which is in turn coupled by a wire 160a, FIG. 6, to the heatexchanger 102. A further connection point E22 is coupled to the heatexchanger 102 by a further wire 160b. The wires 160a, 160b are solderedto the connection points E19, E22 and are bolted to the heat exchanger102.

The conductive trace 120, FIG. 10, connects the diodes D2+, D3+ and theresistor RB1+ to a connection point E12. The connection point E12 isalso coupled by the conductive plate 148 and a pair of conductivemounting posts 162a, 162b, FIG. 6, to the base or control electrode ofthe driver transistor QD+. The mounting posts are secured to thesubstrate 100 and are soldered to the conductive plate 148 at points 164to securely hold the emitter of the transistor QD+ in electrical contactwith the heat exchanger 102.

The conductive traces 126, 128, 130 and 134, FIG. 10, connect the diodesDB1+ through DB3+ in series between the connection point E12 and a pairof connection points E3, E4. These latter two connection points arecoupled to the line 70.

The conductive trace 134 also connects the resistor RB1+ to a pair ofconnection points E20 and E21. These connection points are coupled tothe bases of the transistors Q1+ and Q2+ by the conductive plates 144,146, FIGS. 6 and 7. The emitter of the transistor QD+ is coupled to thebases of the transistors Q1+ and Q2+ by soldering or otherwiseelectrically connecting the conductive plates 144, 146 to an emitterelectrode 169 of the transistor QD+, as seen in FIG. 6.

The conductive traces 132, 136 connect the diodes DB4+ and ZB1+ betweenthe connection point E21 and the emitter bus 108. The conductive trace136 connects the emitter bus 108 to connection points E5-E8, E13, E14and E16. In addition, a connection point E9 is coupled to the emitterbus 108 and a connector 170 is mounted on the board 80 which is in turncoupled to the phase output terminal 38. A connection point E24 is alsocoupled to the emitter bus 108.

As seen specifically in FIG. 6, emitter electrodes 176, 178 of thetransistors Q1+ and Q2+, respectively, are coupled by electricallyconductive mounting posts 180, 182, respectively, to the emitter bus108.

Referring again to FIGS. 7 and 10, the conductive pad 142 connects theanode of the transistor DS+ to the heat exchanger 102. A cathode plate183 of the diode DS+ is coupled by a pair of electrically conductivemounting posts 186a, 186b to a pair of connection points E17. Theconnection points E17 are in turn coupled to the capacitor buses 110,112. The capacitor CS1+ is soldered into via holes 190, 192 so that thecapacitor CS1+ is connected between the capacitor bus 110 and theemitter bus 108. In like fashion, the capacitor CS2+ is connectedbetween the capacitor bus 112 and the emitter bus 108 by soldering theelectrodes thereof into via holes 194, 195.

The holes 190 and 194 are plated through so that the capacitors CS1+ andCS2+ are connected by the buses 110, 112 to the etched foil resistorsRS1+ and RS2+. As seen in FIG. 9, the resistor RS1+ includes terminals196, 197 which are connected between connection points E17 and E22whereas the resistor RS2+ includes terminals 198, 199 which areconnected between the connection points E17 and E19.

The cathode of the diode DF2+ is connected by the electricallyconductive pad 140 to the heat exchanger 102. An anode terminal 200 ofthe diode DF2+ is coupled by an electrically conductive post 204, FIG.7, to the emitter bus 108.

A connector 206 comprises a part of the heat exchanger 102 and comprisesa connection point E10. In the case of the board 80, the connector 206provides a means for connecting the DC link conductor 20a to the heatexchanger 102.

An anode of the diode DN1+, FIGS. 7 and 10, is coupled by the conductivepad 104 and a jumper 210 to the connection point E24. A cathodeelectrode 212 of the diode DN1+ is coupled to an electrically conductivemounting post 214 which is shown in detail in FIG. 7. The connectionpoint E15 is in turn coupled to the input/output bus 109, as areconnection points Ell and E18. As seen in FIG. 10, a connector 220 iscoupled to the input/output bus 109 to allow the board 80 to beconnected to the board 84.

The capacitor CDN2+ is soldered into via holes 222 and 224 which, asseen in FIG. 9, extend through the emitter bus 108 into the thermofoilresistor layer. The via holes 224 are not plated fully through the board80 so that the pins of the capacitor CDN2+ soldered therein do not makeelectrical contact with the emitter bus 108. These pins of the capacitordo, however, establish electrical contact with a terminal 228 of theetched foil resistor RDN2+ so that the capacitor CDN2+ is coupledbetween the emitter bus and such resistor.

A second terminal 230 of the resistor RDN2+ is coupled to the connectionpoint E18, which is in turn coupled by the input/output bus 109 to theconnection point E11.

Referring now to FIG. 11, the board 82 for the negative switch assembly36 is identical to the board 80 with the following exceptions. Elementscommon to FIGS. 10 and 11 are assigned like reference numerals with theexception of electrical components wherein a minus sign is substitutedfor a plus sign, as noted previously. The diode DN1- is turned over ascompared with the diode DN1+ so that the anode of the diode DN1- iscoupled to the connection point E15. Further, the electricallyinsulating spacer 105 is not used so that the cathode of the diode DN1-is coupled electrically and thermally to the heat exchanger 102.

The jumper 210 is not utilized in the negative switch assembly of FIG.11. Rather, a jumper 230 connects the electrically conductive pad 104,and hence the cathode of the diode DN1-, to a connection point E23 whichis in turn coupled to a first terminal 232 of the etched foil resistorRDN1-, seen in FIG. 9. A capacitor CDN1- includes pins which extendthrough via holes 236 to connect to a second terminal 238 of theresistor 20 RDN1-. A second set of pins of the capacitor CDN1- extendthrough via holes 240 to establish electrical contact with theinput/output bus 109.

The capacitor CDN2+ is not used in the negative switch assembly 36. Byomitting this capacitor, the resistor RDN2+ is likewise not used.

Referring now to FIG. 12, the circuit board 84 forming a portion of theneutral switch assembly is illustrated in greater detail. Again,elements common between FIGS. 10 and 12 are assigned like referencenumerals, with the exception that electrical components are identifiedby the suffix N instead of the plus sign. As before, only thedifferences between the boards 80 and 84 will be described in detail.

The insulating spacer 105 is not used in the board 84 so that thecathode of a diode DN1N is coupled to the heat exchanger 102. Instead,the electrically insulating spacer 105 shown in phantom in FIG. 7 isplaced between the conductive pad 140 and the heat exchanger 102 so thatthe cathode of a diode DN2N is insulated therefrom. An electricallyconductive jumper 252 connects the cathode to the connection point E18.This connection point is in turn coupled by the input/output bus 109 toa capacitor CDN1N and an anode electrode 254 of a diode DN1N via themounting post 214. The diode DN1N is turned upside down as compared withthe diode DN1+ of the board 80 so that the cathode electrode thereoffaces and is in electrical contact with the heat exchanger 102. Thecathode electrode of the diode DN1N and the heat exchanger 102 arecoupled by the conductive pad 140 and the jumper 230 to the connectionpoint E23.

The capacitor CDN1N is coupled between the connection point E15 and theresistor RDN1N. A capacitor CDN2N is coupled to the anode of the diodeDN2N by the emitter bus 108 at the connection point E16. The resistorRDN2N is connected between the capacitor CDN2N and the connection pointE18.

Referring again to FIG. 4, the boards 80, 82 and 84 are interconnectedby wires or other conductors such that the connection point E11 of theboard 80 is connected to the connection point E10 of the board 84 usingthe connectors 220 and 206 and so that the connection point E11 of theboard 82 is connected to the connection point E9 of the board 84 usingthe connectors 220 and 170. The phase output terminal 38 is coupled tothe connection point E24 of the board 80 via the connector 170 and tothe connection point E10 of the board 82 via the connector 206. The DClink conductor 20a is coupled to the connection point E10 of the board80 using the connector 206. The DC link conductor 20b is coupled to theemitter bus 108 of the board 82 using the connector 170. The neutralterminal 32 is coupled to the connection point E11 of the board 84 bythe connector 220. The lines 64, 66, 68, 70, 72, 74, 77 and 79 arecoupled to the respective boards 80, 84 using wires or other conductorswhich are soldered or otherwise electrically connected to the respectiveconnection points thereof.

If desired, the boards 80, 82 and 84 may be arranged in a stackedconfiguration with electrically insulating sheets or spacerstherebetween which may be made of Kapton or another suitable material.In such case, a threaded bar or rod (not shown) may extend throughaligned holes 270 in the boards, 80, 82 and 84 and through aligned holesin electrically insulating end caps (not shown) which are disposedadjacent the outer faces of the resulting stacked boards 80-84. Nuts(not shown) may be threaded on to the threaded bar to move the end capstoward one another to thereby force the diodes and capacitors intointimate contact with the heat exchangers 102 so that good electricalcontact is made therebetween.

As seen in FIG. 7, the heat exchanger 102 includes a passage 280 havinga plurality of cooling fins 282 disposed therein. Cooling oil or othercooling fluid enters one of two ports 284, 286 and exits the other port.When the boards 80, 82 and 84 are assembled in stacked relation, theports 284, 286 of the boards 80, 84 may be interconnected so that theheat exchangers 102 are effectively coupled in parallel to receivecooling medium.

FIGS. 8 and 9 illustrate the construction of the substrate 100 ingreater detail. The main layer 101 comprises the buses 108, 109, 110 and112 together with the filler 114 and the epoxy. A first or upper side ofthe main layer 101 is covered with an electrically insulating coating280 which has been etched as required, for example at areas 282. Theelectrically conductive traces 120-136 are then formed on the insulatinglayer 280 and a further insulating layer 288 is formed atop the traces120-136. Again, the layer 288 is etched where necessary to allowconnections to the traces and to other electrical components of theboard.

As seen in FIG. 9, a still further electrically insulating layer 290 isdeposited on a second or lower side of the first layer 101, again withportions etched where necessary. A layer of electrically resistivematerial is then deposited on the layer 290 and portions thereof areetched to form the etched foil resistors. Finally, an electricallyinsulating layer 292 is deposited over the etched foil resistors toinsulate same. The layer 292 is etched where necessary to allowconnections between the electrical components of the various layers.

As seen in the Figures, the main layer 101 occupies the majority of thethickness of the substrate 100 so that the buses 108, 109, 110 and 112and filler 114 extend substantially the entire distance between firstand second faces of the substrate 100. These portions provide mechanicalrigidity for the circuit boards 80, 82 and 84.

In the preferred embodiment, the value and rating of correspondingcomponents on the boards 80, 82 and 84 are the same so that the boardscan be manufactured and shipped with such components thereon. Such aboard blank may then be configured for a positive switch assembly 34 byadding the insulating pad 105, the jumper 210 and the capacitor CDN2+,as noted in greater detail above. The board could alternatively beconfigured as the negative switch assembly 36 by adding the jumper 230and the capacitor CDN1- and by reversing the diode DN1 - as comparedwith the diode DN1+ or may be configured as a neutral switch assembly 40by adding the jumpers 230 and 252, the insulating pad 250 and thecapacitors CDN1N, CDN2N and by reversing the diode DN1N as compared withthe diode DN1+.

It can thus be seen that a "generic" board or board blank can beprovided with components mounted thereon which are common to thepositive, negative and neutral switch assemblies, together withadditional components which would allow the board to be configured asone of the positive or negative switch assemblies or as a portion of theneutral switch assembly. This desirable standardization simplifiesreplacement of boards containing one or more faulty components andobviates the need to stock separate replacement boards for each switchassembly. A significant reduction in inverter down time and inventorycost may thus be realized.

We claim:
 1. A circuit board for an inverter, comprising:a substrateincluding a main layer having two separate electrically conductivebuses, first and second layers of electrically insulating materialdisposed on first and second sides of the main layer, respectively, aseries of traces disposed on the first layer of electrically insulatingmaterial, a third layer of electrically insulating material on theseries of traces, a planar resistor disposed on the second layer ofelectrically insulating material and a fourth layer of electricallyinsulating material disposed on the planar resistor; a plurality ofresistors and diodes interconnected by the traces and disposed atop thethird layer of electrically insulating material; a capacitor disposedatop the fourth layer of insulating material and electrically coupled onone of the substrate buses between the planar resistor and one of thesubstrate buses; an electrically conductive heat exchanger disposed onthe fourth layer of electrically insulating material and coupled to theplanar resistor; and a power transistor and a power diode both inelectrical and thermal contact with the heat exchanger and further inelectrical contact with the other of the buses of the main layer.
 2. Theswitch assembly of claim 1, including a further capacitor and a furtherplanar resistor coupled together in series between the heat exchangerand the other of the buses of the main layer, the further capacitorbeing disposed atop the fourth layer of insulating material and theplanar resistor being disposed between the second and fourth layers ofelectrically insulating material.
 3. The switch assembly of claim 1,wherein the heat exchanger includes first and second faces wherein thepower transistor is in contact with the first face and the power diodeis disposed within an aperture through the substrate in contact with thesecond face.